Hydrogen-enriched Compressed Natural Gas (HCNG) is a clean alternative fuel that combines the advantages of natural gas and hydrogen fuels for motor vehicle engines. Hydrogen enrichment improves the low burning velocity and poor combustion stability of Natural Gas fueled engines. Natural Gas fuel has generated much interest as an alternative fuel due to its potential for low particulate and hydrocarbon emissions. HCNG fuels provide advantages over Natural Gas by increasing efficiencies, power output, and further reducing emission through engine controls modifications.
Hydrogen is the most abundant element in the universe and is considered by the scientific community as the ideal alternative fuel. However, the present lack of hydrogen infrastructure, including production, distribution, and storage, and the high capital cost of developing that infrastructure has made the widespread use of hydrogen fuel economically unfeasible.
An effective solution for overcoming the structural barriers to the use of HCNG fuel is the on-board generation of hydrogen in motor vehicles through a natural gas reforming system utilizing engine exhaust gases. The utility of this system overcomes the costs, inefficiencies, and safety hazards associated with the production, distribution, and storage of hydrogen fuels.
Natural gas expressed in mole fraction is typically 95% methane. Other components include less than 2% ethane, propane, and less than 1% inert gases such as carbon dioxide and nitrogen. Raw natural gas requires processing to remove impurities, including water, to meet industry specifications for marketable natural gas.
Hydrogen is produced by a number of different processes including water splitting, electrolysis, and separation from industrial waste streams. Hydrogen can also be produced through reforming natural gas. A reformer is a form of fuel processor that converts hydrocarbon fuels including methane, propane and natural gas into hydrogen. The majority of commercially available hydrogen is generated through steam-methane reforming. Typically, a multi-step process is used to produce a high purity hydrogen gas stream, which can be used for a variety of purposes including mixture with other gases to produce an alternative fuel.
The most common form of reforming employs the use of steam (H2O) and a hydrocarbon fuel. The hydrocarbon fuel is reacted in a heated reaction tube containing steam (H2O) and at least one other catalyst. The primary derived reaction in the steam reformer is an Equilibrium Reaction (1) as indicated:CH4+H2OCO+3H2  (I)
As Equilibrium Reaction (I) moves to the right 2 moles of gas are converted to 4 moles of gas. This causes the reaction to be highly endothermic (−198 kJ/mol) and demonstrates pressure sensitivity (Le Chatelier's Principle) as hydrogen production is enhanced at lower pressures.
All four of the substances in Equilibrium Reaction (I) exist in the reformer as a gas mixture with excess steam (H2O). In addition to the primary products of CO and H2, a secondary Equilibrium Reaction (II) occurs:CO+H2OCO2+H2  (II)
Equilibrium Reaction (II) is the Water Gas Shift Reaction. The reformer contains five gases in varying concentrations according to the equilibrium constants for Reactions (I) and (II). These equilibrium constants are temperature sensitive (see FIG. 1).
A separate Shift Reactor operates at a lower temperature to enhance Equilibrium Reaction (II). The overall objective of the reforming and shift reactions is to maximize hydrogen production.
Four other gases are present in varying concentrations and are impurities that must be removed in order to produce high purity hydrogen (H2). The passage of methane (CH4) through the process without undergoing reaction is known as “methane slip”. For most hydrogen applications methane slip and carbon monoxide (CO) are impurities that must be removed. Fuel cell hydrogen requires a level of purity that dictates additional steps to remove relatively inert methane (CH4) and carbon dioxide (CO2), and carbon monoxide (CO). Otherwise, hydrogen impurities degrade fuel cell performance and catalyst life. The gas stream exiting the shift reactor also contains water vapor (H2O) which must be removed by a condenser before further purification measures are applied.
In order to make high purity hydrogen (H2), a final pressure swing adsorption (PSA) process may be performed. The PSA process involves a high pressure adsorption of impurities from the hydrogen (H2) onto a fixed bed of adsorbents. The impurities are subsequently desorbed at low pressure into an off-gas stream, thereby producing an extremely pure hydrogen gas (H2). For example, product purities in excess of 99.999% (H2) by volume percentage can be achieved. The off-gas stream, which includes carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) plus small amounts of water vapor and hydrogen (H2), is returned to the process as supplemental fuel.
A process for producing hydrogen through steam-methane reforming is shown in FIG. 2. In Step 1 a hydrogen-rich gas stream is produced by injecting methane (CH4) and steam (H2O) into a reformer where it is reacts in the presence of a catalyst. Step 2 moves the hydrogen-rich gas stream through a shift reactor where carbon monoxide (CO) reacts with steam to produce additional hydrogen. Both Steps 1 and 2 are endothermic reactions requiring a heat source. Step 3 is a condensing step to remove most of the water vapor (H2O) from the hydrogen-rich gas stream. Step 4 is compression step where the hydrogen-rich gas is compressed to a specified pressure. Step 5 is the PSA step to remove impurities from the hydrogen-rich gas stream. The impurities include carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), and residual water vapor (H2O) which, in addition to small quantities of hydrogen (H2), are recycled back to the boiler and/or auxiliary burners (not shown). It is also general practice to recover waste heat throughout the process with various heat exchangers (not shown).
The traditional methods of producing high purity hydrogen gas has required significant capital investment in compressor and PSA columns as well as operating expenses to supply electric power for the compressor. The PSA apparatus is comprised of vessels and valves connected and separated through conduits that have been difficult to reduce in size.